1. Technical Field
The present invention relates to an eddy current type retarder for a vehicle and particularly to a retarder having a permanent magnet as a magnetic source.
2. Background of the Art
A retarder employing eddy current is generally known as an axiliary brake system for a vehicle to ensure safety braking. Such a retarder is particularlly useful when the vehicle is running on a long hill, for example, since a main brake (foot brake) is sometimes not enough to ensure sufficient and safe deceleration.
A typical retarder includes a rotor mounted on a rotary element (generally a propeller shaft or a shaft extending from a transmission) which rotates with wheels and a magnetic source (electromagnets or permanent magnets) mounted on a stationary element (generally a frame of the vehicle). Eddy current is produced by a relative speed difference between the magnetic source and the rotor and the eddy current produces brake force to the rotary element.
FIG. 7 of the accompanying drawings shows an eddy current type retarder (x) owned by the present assignee (Japanese Patent Application No. 1-218499 and not published yet). This retarder uses permanent magnets as the magnetic source. As illustrated, an output shaft (a) extends from the back of a transmission housing (c) and a hollow drum-shaped rotor (b) is mounted on the output shaft (a). Permanent magnets (d) extend in the rotor (b). The magnets (d) are indirectly mounted on the transmission housing (c). Each magnet (d) is movable in an axial direction of the drum-shaped rotor (b) or an axial direction of the shaft (a). In other words, each magnet (d) can approach and leave the rotor (b). The permanent magnets (d) are arranged at certain intervals on a support ring (e) such that the magnets (d) face the inner wall of the rotor (b) in the circumferential direction of the rotor (b). The poles of the magnets (d) are reversed in the direction they are arranged, i.e., if an N pole of one magnet faces the inner wall of the rotor (b), an S pole of an adjacent magnet faces the inner wall.
When the retarder (x) is operated to apply the brake force to the vehicle, the support ring (e) is moved to the right in FIG. 7 by an actuator (f) such as an air cylinder as indicated by the solid line in illustration. Consequently, the permanent magnets (d) approach the rotor (b). Then, magnetic circuits are formed between two adjacent magnets (d) on the stationary member (e) and the rotating member (b), and eddy current flows in the inner wall of the rotor (b). The eddy current imposes the brake effort on the output shaft (a) and the vehicle is decelerated.
When the brake is released, the support ring (e) or the magnets (d) are moved in FIG. 7 by the actuator (f) as indicated by the broken line. As a result, the magnets (d) leave the rotor (b) and the magnetic connection between the magnets (d) and the rotor (b) is cut. At this situation, the brake force is not applied to the output shaft (a) since the eddy current no longer flows in the rotor (b).
Another eddy current type retarder (y) is illustrated in FIG. 8. This retarder is disclosed in a Japanese Patent Application of Sumitomo Metal Co., Ltd., published Dec. 1, 1989 with the publication No. 1-298947. In this brake system, a drum-shaped rotor (h) is mounted on a rotary shaft (g) and permanent magnets (i) are positioned in the rotor (h). The shaft (g) is connected with wheels (not shown). The magnets (i) are mounted on a support ring (j) at predetermined intervals and face the inner wall of the rotor (h). The magnets (i) are mounted on the support ring (j) such that the poles of the magnets which face the inner wall of the rotor (h) are alternately reversed. The support ring (j) is mounted on a stationary member (not shown). The support ring (j) is adapted to rotate about a shaft (g) within a certain angle range. Between the permanent magnets (i) and the rotor (h), there are provided ferromagnetic elements (k) and non-magnetic elements (1). Both elements (k) and (l) are mounted on the stationary member. The ferromagnetic elements (k) are provided at predetermined intervals such that each ferromagnetic element (k) can be selectively faced by the corresponding permanent magnet (i). The non-magnetic elements (l) are located between each two adjacent ferromagnetic elements (k).
When the retarder (y) is operated to apply the brake force to the shaft (g), the support ring (j) is rotated to a position shown in FIG. 8 such that the respective ferromagnetic elements (k) face the corresponding permanent magnets (i). As a result, magnetic circuits (m) are formed between the rotor (h), each two adjacent ferromagnetic elements (k) and each two adjacent magnets (i). Thus, eddy current is produced in the inner wall of the rotor (h) as the rotor (h) rotates, and the rotation of the shaft (g) is decelerated.
When the brake force is released, the support ring (j) is rotated about the shaft (g) to a position as shown in FIG. 9. Specifically, the ring (j) is moved clockwise or countercloskwise in a manner such that each ferromagnetic element (k) faces vacant space between the magnets (i). In this case, as illustrated, since the ferromagnetic element (k) is longer than the magnet (i), the ends of the ferromagnetic element (k) still face the ends of the magnets (i). Accordingly, different types of magnetic circuits (n) are formed between the ferromagnetic elements (k) and the magnets (i). However, the magnetic circuits (n) do not reach or penetrate the rotor (h) in this case. Therefore, the eddy current is not generated in the rotor (h). The ferromagnetic elements (k) serve as the magnetic shield at the brake releasing operation.
Meanwhile, the above-described retarders (x) and (y) have following disadvantages.
In the brake device (x) of FIG. 7, since the brake force is controlled by the movement of the permanent magnets (d) in the axial direction of the rotor (b) in the hollow part of the rotor (b), the brake device (x) requires a large space (l.sub.1) (the sum of space for the movement of the magnets (d) and space for the acutator (f) in the axial direction of the rotor (b), as shown in FIG. 7. Thus, when the brake device (x) is mounted on the shaft (a) extending from the back of the transmission housing (c), the rotor (b) overhangs toward a propeller shaft joint (p). The overhanging rotor (b) becomes an obstacle to a mechanic when he removes and assembles the propeller shaft. Particularly, if the brake device is mounted on a truck, a cross member of a frame of the truck extends near the brake device so that the probelm becomes more serious.
In the brake device (y) of FIG. 8 or 9, the magnets (i) mounted on ring (j) are moved about the shaft (g) to control the brake force. Consequently, the brake device (y) requires space smaller than the brake device (x) of FIG. 7. In other words, the brake device (y) can be designed compact as compared with the brake device (x). However, in the brake releasing operation of the brake device (y), some magnetic fluxes from the magnets (i) penetrate the non-magnetic elements (l) since the non-magnetic elements (l) are relatively thin, as indicated by the broken line (q) in FIG. 9. The magnetic fluxes which have penetrate the non-ferromagnetic elements (l) reach the rotor (h) and form magnetic circuits (q) between the rotor (h) and the magnets (i). As a result, a small amount of eddy current flows in the rotor (h) even in the brake releasing operation. In other words, magnetism leakage occurs in the brake releasing operation. This lowers the fuel consumption rate of the vehicle.